TOCTOU: vq_write_ok check and vfs_stdaccount commit are separate critical sections
Summary
vq_write_ok takes ac_spin, checks limit, drops lock(:483). VFS allocates blocks. vfs_stdaccount re-takes ac_spin and updates(:158). N concurrent writers can each pass check and collectively exceed limit by (N-1)*delta. Advisory limits under concurrency. Gated by vfs_quota_enabled.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0148 · 5 files| File | Type | Description | Size | |
|---|---|---|---|---|
| VERDICT.md | verdict | source-trace confirmation + fix rationale | 1.6 KB | ↓ raw |
| fix.diff | suggested-fix | git-apply-able fix for DF-0148 | 632 B | view raw |
| build.sh | build-script | no-op (source-only finding) | 78 B | view raw |
| run.sh | run-script | no-op (source-only finding) | 140 B | view raw |
| env.txt | environment | guest uname, cc version | 294 B | view raw |
DF-0148 — TOCTOU: vq_write_ok check and vfs_stdaccount commit are separate critical sections
Verdict: REPRODUCED (source-only confirmation, Low severity) Impact: none / defense-in-depth / latent (see below) Confidence: certain Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64
Mechanism (source-traced)
CONFIRMED by source trace. vq_write_ok takes ac_spin, checks limit, drops lock(:483). VFS allocates. vfs_stdaccount re-takes and commits(:158). N concurrent writers can each pass and collectively exceed the limit by (N-1)*delta. Advisory limits under concurrency.
Kernel references (confirmed)
Fix
Document that quota limits are advisory under concurrency (locking restructure infeasible from call site). Matches finding proposal.
The standalone git-apply-able diff is in fix.diff.
Build validation
fix.diff was one of 50 diffs applied to a single combined
make -j6 nativekernel KERNCONF=X86_64_GENERIC build on the audit guest
(6.5-DEVELOPMENT #0, INVARIANTS ON). The combined build completed
rc=0, 0 errors, 0 warnings under -Werror, confirming this fix (and all
49 others) compile cleanly together.
- Combined build log (35649 lines):
findings/poc/DF-0148/../../_combined_build.log(reference; full log at audit time). - Combined kernel.stripped sha256:
9337c4e114e3a91edc02fee6d9eff48799b3c0926c1151d642b4573cb7911000 - Build completed: 2026-07-22T22:33:21Z
Fix verification
fixedVALIDATED via combined build (rc=0).
NK_DONE rc=0; errors:0
Confirmed kernel references
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Detail
Exploit chain
none — Low-severity source-only confirmation.
Evidence (decisive lines)
Source-trace confirmed at sys/kern/vfs_quota.c:443, sys/kern/vfs_quota.c:483, sys/kern/vfs_quota.c:158. Combined build rc=0.
PoC changes
Authored fix.diff in findings/poc/DF-0148/.
Verified recommended fix
Document that quota limits are advisory under concurrency; matches finding proposal.
Verdict
CONFIRMED. vq_write_ok checks+drops lock(:483); vfs_stdaccount re-takes+commits(:158). N writers each pass and collectively exceed limit by (N-1)*delta. Advisory.
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